Art of firing ceramic and other products



C. T, HOLCROFT ET AL ACT OF FIRING CERAMIC AND OTHER PRODUCTS Filed May12, 1924 March 8 1927.

Patented Mar. 8, 1927.

UNITED STATES PATENT OFFICE.

CHARLES-T. HoLcItoFT, or DETROIT, ann` CHARLES Iz. nom., or lMOUNT.cLEMENs MICHIGAN.-

ART OFFERING-CERAMIC AND OTHER PRODUCTS.

Application led Hay 12, 1924. Serial No. 712,561.

This invention relates to improvements in the art of firing ceramic andother products and also to the firing of decorated ware for the purposeof fusing the applied materials thereon." y

In roducing pottery ware thev process genera ly embodies three stages.

The `first stage produces the bisque product, this stage taking the rawmaterials, forming them into the desired shape, drying the shapedarticles to eliminate the major portion of the moisture, and then firingthe dried embryo article; in the firing step of this stage the remainingmoisture 1s first driven off, after which comes the development of thechemical reactions that take place as the applied temperature increases,these reactions becoming' complete as the embryo ware reaches the peakvtemperature of the stage (approximatel 2400o F.) to be succeeded by thecooling o the Ware until it is cooled sutciently to permit properhandling. It may be noted that in referring to temperatures, referenceis made to the temperatures in the kiln.

The second stage involves the application of the glaze. The embryo pieceof Ware (dinner Ware, for instance) receives a coating of the materialwhich is to produce the glaze, is subjected to a drying action, and thenis fired to fuse and set the glaze. In the firing step of this stage thepeak temperature is approximately 2200 F., this peak being reachedgradually, the ware then being permitted to cool gradually, completingthe piece of ware with the exception of its decorations.

The third stage bears on the decorating activities, the decorationsbeing applied onto the glaze, after which the Ware 1s subjected to athird tiring action to fuse the decoration upon the Ware. In this stagethe peak temperature in' the kiln is approximately l500 F., the,l Warebeing gradually brought to this temperature and then cooled to producethe finished product. It is thel firing steps of these stages that formthe primary subject-'matter of the present invention.

As will be understood, the ware of each stage is subjected to hightemperature conditions` the first firing presenting the highest peak,the third firing presenting the lowest peak; but it is obvious, ofcourse, that in the secondl stage the ware will pass through yAs thetemperature rises,

many of the characteristics of the vrst stage up t'o the peaktemperature lof the second stage-the difference being in the fact thatthe chemical reactions of the bisque product were .completed during theIfirst stage, the second stage dealing more particularly with thereactions that take place on the surface of the ware. And this is truealso with respect tothe third tiring Wit-hin the limits of the peaktemperature of the' third stage.

I-Ience, the Ware is carried to the peak of.

the third stage in each of the stages; it is carried to thepeak of thesecond stage during thearst and the second stages, and to the peak ofthe first stage duringthis stage. v

'This is referred to more particularly to polnt out the conditions of afundamental problem that is'found in connection with the tiringoperations of the third stage, this being one of a number of problemsthat are required to be correctly solved in order to produce a properand complete product. This problem will be understood b a briefcomparison of characteristics o the different tiring operations.

One of these characteristics- Which can be used as a tell-tale-is foundin the change in color ,of the Ware at diferent temperatures. the warepasses first into what is termed a black color, this being simply anexpression used to indicate the fact that inspection of the ware withinthe kiln presents no levidence of the presence of heat in the Warealthough heat is present; this is most noticeable during the descendingtemperature conditions. As the temperature rises, the Ware passes to adeep red color, from which it passes into a llight red color, andfinally into a yellow-red color, this latter occurring when the`temperature rise approaches its peak during the firing of the firststage, the peak of this stage presenting this as the color of the ware;a higher temv what is termed a tender or a critical zone which includesa range of between 300 F. and 16000 F., the latter being within the deepred color zone with the former in the black zone. Within thesetemperatures strains and stresses are present in the ware, this being azone in which the ware is not in plastic form but still subject to theconditions of expansion and contraction-expansion under ascendingtemperature conditions, and contraction under descending temperatures.During this period shattering of the ware is always possible; too rapidtemperature rise or a too rapid temperature drop within this zonepractically ensures shattering of the ware.

Since this zone is present in each stage the danger is present with eachfiring step. In the first stage the ascending temperatures are not aslikely to produce this result, but it is present during the descendingtemperature conditions; it is present under both ascending anddescending conditions in the second and third stages. This tender zonecondition and its temperature range present one of the controllingdifferences between the activities of the stages.

As will be understood, the ware is carried to the peak temperature of astage by anascending temperature condition, whereupon the ware passes tothe descending temperature conditions to ermit cooling. During thisparticular perio of passing the temperature peak the -ware is subjectedto the change from the conditions present at the ending of the expansionperiod to those which begin the contraction period. Prior to reachingthis peak zone, the action has all been in the same direction-ascendingtemperatu-res leading to expansion; passing the peak, this uniformity indevelopment is changed to the descending direction and the contractingaction begins to continue until the ware is cooled.

In the rst and second stages, this change from ascending to descendintemperature conditions takes place well a ove the temperatures of thetender zone; in both stages, the tender zone is an intermediate part ofthe uniform development of temperature conditions. The tender zone ispassed long before the ascending temperature condition ends, and thedescending temperature uniformity has become well established beforethis zone is reached during the cooling operation.

The conditions, however, are dilferent in the third stage. Here the'peak is itself Within the tender or critical zone, so that the chan ewhich ends the expansion condition and egins the contraction of theware, takes place within this zone; the change is close to the up rlimit of the range of this zone, so that t e major portion of theheating and cooling activities take place within this zone-only at thebeginning of the heating and the ending of the cooling periods is vat alower' temperature than that of the mineral colorssuch as are applied bythe decalcomania process, for instance; these latter generally ffuse 'atthe peak temperature indicated. If higher temperatures are used thecolors become damaged and the ware decorations do not present thedesired beauty nor are they lasting. The metal colors, such as gold, arenot absorbed wit-hin the glaze-they are fused on the glaze, and

-the lasting qualities of the decoration are determined by the manner inwhich the ware is fired. On the other hand the mineral colors areabsorbed by the glaze; if an insufficient heat is used, the absorptionis incomplete and the colors can be Washed oli', or fail to developproper shade of color.

The metal and the mineral ingredients require different fusingconditions; and this leads to one of the several problems present in thefiring of decorated ware under quantity production conditions. A heatthat -is proper for'the gold stripe for instance, will leave the mineralcolors insuficiently fused; and a heat for the mineral colors will causedeterioration of the gold. In general practice, therefore, the two typesof ware are fired separately in order to avoid these objectionableconditions; and this, in turn, tends to ai'ect production.

Decorative ware firing is generally provided by the use of the periodictype of muflle kilns. lTunnel kilns have been employedl for the firingstep in the first and second stages, but these have not been generallyemployed in connection with the third or decorative tiring stage. In thefirst and second stages the ware is carried in saggars and the carscontaining the latter are carried through the heated tunnel, thussetting up' conditions of a inutile action. but the. firing is donc instagnant air. since the saggars are designed 'to keep out thc productsof combustion found in the tunnel kilns. A further reason, however, isthe fact that it is desired to keep the Wale from sudden changes intemperature during thc range of the tender zone, so as to prcvcntshattering of the ware.

In the muiile type of periodic kiln, the

Inn

Leaopaa4 tive ware firing-fthe deteriorating effect of the products ofcombustion on the decorating materials. On an ascending heat, expansionis present in the Walls Within which the Ware is located, and hencethere is likeli hood of leakage of the products through the walls duringthe earlier heat ranges; and the same conditions apply on the descendingheats. Since the firing is periodic-the kiln is filled `With the Ware tobe fired', and remains in this condition until the firing cycle iscompleted-this alternate condition of expansion andk contraction ofWalls is present with each firing operation, and hence there is alikelihood of air contamination in the space carrying the Ware throughleakage of the products of c'ombustion.

One ofthe fundamental teachings and experiences that has been presentthroughout the commercial activity of pottery firing and the like, isthat the firing must be done in stagnant air-air is not to be admittedto the chamber in which the firing is to take place, owing to the edectit would have in setting up the .conditions of shattering the Ware. Itwas Well-known that if the firing could be done in thepresence ofcomplete oxygen conditions such as would be provided by bathing the Warewith fresh air during decorative firing, the oxidation of the colorswould be far superior and a more beautiful decoration Would result. Butthis Was considered as simply a dreaman ideal that 'was impractical-dueto the fact that the tendency to shatteringvvaslargely increased by theattempt to `supply the air.

These facts provide a setting in which to consider the present inventionwhich is designed to provide for the firing of ceramic wareV and kindredproducts, with the activities provided under quantity productionconditions. Under the present invention the firing is carried on undercontinuous instead of periodic-operation conditions, and in thedecorative firing stage, with the Ware subjected to the firing cycleregardless of the character of the decorative materials.

In carrying out the methods of the present invention the ring isprovided within a kiln or oven of the tunnel type, the kiln or ovenbeing constructed on muiiie-action conditions. In addition, the methodincludes that which, heretofore, has been believed to be impossible ofsuccessful operation-the ware is fired while bathed with air introducedinto the firing chamber, being present and circulated through the tunnelthroughout the length of travel of the ware through the tunnel. Not onlyis the air present but it is present under a pressure suilicient tocounteract the pressures of the products of combustion outside of themuiile chamber, with the result that leakage of products of combustionto the muie chamber is substantially eliminate-d. Further, in order tomaintain a constant even pressure in all parts of the muille chamber,throughout its length, the cross-sectional area of the chamber isincreased at those parts of its length Where the temperature increasesand is contracted Where the temperature is decreased, that is thecross-sectional area is varied in proportionto the temperature andconsequent volume of air to maintain a substantially even internal airpressure in the chamber throughout its length.

An underlying characteristic of the invention is the discovery by usthat it is possible to bathe the Ware in air circulated through 85 themuiiie chamber under pressure, where the air and Ware have temperaturesthat vary from each other but in Which the variation is more or lessuniform as -to extent, and is Within reasonable limits. by maintaining acomparatively constant variation during the periods of increasing anddecreasing tempe-ratures, it is possible to provide for crossing thepeak zone-#With In other Words,

its change in direction ofactivity-vvithout setting up the tendency ofshattering the ware; with the variation between the temperature of wareand air of relatively small amount, the changes which take place in thepeak zone and Which have the effect of practically bringing the air andWare to an approximately equal temperature Within the peak zone, aremade gradually and With `approximate uniformity, so that sudden changesare eliminated.

This discovery makes is possible to provide decorative firing underoxidizing con-' ditions; and it has the additional advantage, in thisrespect, of permitting all firing op-` erations to take place underthese conditions and under the conditions of a comparatively shorttunnel kiln as compared with those designed for cooling under naturalcooling conditions. In addition, the discover enables tunnel kilnoperation underoxi izing temperature conditions, permitting kilnoperation With higher temperatures and yet Without requiring the use ofkilns of an -impracticable length.

An additional feature of the invention is the saving in fuelconsumption. In an installation in which the invention is in actuai use,it has been found that the cost of fuel is not more than 25% of the fuelcost of periodic muffle kilns designed for similar purposes. Inaddition, the time required to fire the ware is very materially reduced.

Other features that are advantageous are:

Uniformity of temperature as between the sides and the top and bottom ofthe ware carrier, providing for equal firing action on all of the warecarried.

Reduction in losses from spitting out, dunting, or cracking, over-tiringor undertiring, burned gold or'gold improperly set and which rubs olf,and the destruction or deterioration of gold or colors from the stagnantatmosphere in the kiln or from the presence of sulphur, carbon monoxide,or other impure gases.

Giving to gold and colors a smoothness and bright glossy and lustreappearance that cannot be obtained regularly in other types of muiilekilns of either the periodic or continuous type.

As an entirety, the advantages resulting from practice of the method canbe summed up as quantity of production during a given period, quality ofproduction, and economy of production, the latter not only from afuel-saving standpoint, but also because of simplicity of operationwhich permits the use of unskilled labor.

To these and other ends therefore, the nature of.y which will be readilyunderstood as the invention is hereinafter more fully disclosed, saidinvention consists in the improved methods hereinafter more fullydescribed, illustrated in the accompanying drawings, and moreparticularly pointed out in the appended claims.

In the accompanying drawings, in which similar reference charactersindicate similar parts in each of the views,

Flgure 1 is a central longitudinal sectional View of a simplearrangement of kiln designed for the purpose of carrying out the presentinvention;

Fig. 2 is a horizontal sectional view of the same;

Fig. 3 is a temperature graph giving approximately the temperatures ofthe air and the ware at successive points in the length of the kiln efFigs. 1 and 2.

Figs. 1 and 2 illustrate a Simple arrangement of kiln adapted to carryout the fundamental features of the present invention, the arrangementshown being somewhat diagrammatic. A companion application Serial Number753,084 filed Dec. 1, 1924, discloses a kiln structure designed to carryout the present methods, and in which various structural refinements areutilized to enhance the activities under service conditions.

Because of this fact, the present disclosure.

does not include specific details, suflicient being shown to permit ofthe method being practiced by those skilled in the art to which theinvention pertains.

In the drawings, a indicates generallyv a tunnel kiln construction ofsuitable structural formation, the construction being such as to providea tunnel I) whichv extends throughout the length of the` kiln, the endsof the tunnel beingnormally closed by doors l0, these being arranged topermit opening and closing for the admission of ware into and itsdischarge from the tunnel. In the drawings, the Hoor of the tunnel isindlcated at b; since the ware may be carried through the kiln indifferent ways, well known in the art, as by a conveyor structure or oncars travelling on tracks located on the floor, the showing of the flooris diagrammatic to indicate more particularly the dimensioncharacteristics of the tunnel rather than the specific ware-conveyingmeans. As disclosed in the companion application, we prefer to employthe car and track arrangement, the cars being introduced into the endshown at the left in Fig. 1, at spaced intervals of time, lthe tunnelbecoming lilled with cars so that the introduction of one car acts toadvance the entire train of cars, and discharges the advance car throughthe door opening at the righthand end of Fig. 1. This provides for anintermittent advance of the train of cars. This method of carrying theware through the kiln is one that has been employed for many years inconnection with oven kiln and furnace structures as evidenced by thefollowing patents; Bennett, No. 48,761, July 1l, 1865; Gillinder No.337,575, March 9, 1886; Lawton, et al., No. 405,571, June 18, 1889;Fellner, et al., No. 455,191, June 30, 1891, and Gery, No. 793,290, June27, 1905, these being instances of the ware carrier being in the form ofcars which are advanced intermittently through the tunnel. The conveyortype of kiln is indicated, for instance, in the patent to Hanze, N o.952,196, March 15, 1910.

The kiln is of the muiiie type, the heat oenerating means being locatedwithin the offset indicated at c, this being at an intermediate portionof the length of the kiln, and, as shown, is of the type in which oilburners are used for the purpose of producing the heat, it beingobvious, of course, that the heat could be provided in other ways, asfor instance, by the use of coal fires or in any other suitable manner.The offset c may thus be considered as the furnace part of the kiln.

As shown in Fig. 2, the products of combustion from the furnace are ledtoward the left in'said figure, between the outer walls of the kiln andthe muliie walls presently referred to, these walls forming channels cwhich lead toward the entrance end of the kiln and through which theproducts of combustion )ass and finally discharge into a space or ue dleading to a chimney al which provides for the draft conditions,channels c and iue al being in communication adjacent the ends of thechannels at the entrance end of the kiln, so that the products ofcombustion travel practically throughout the length of the channels andlll) are then discharged into flue d to passjout of the chimney.

The mutile wallswithin the Zone of activity of the products ofcombustion, have vertical portions practically in two zones in theelnbodiment shown in the drawing, al-

y though, this may vary. The zone e which reaches from the furnace to anintermediate point toward the entrance end of the tunnel iscomparatively thin, and may be provided, for instance, of suitablesheets of heat radiating material, the thinne'ss of the walls permittingof heat radiation with great facility; the portion of this wall,indicated at e may be and preferably is thicker. The top of the kilnpractically forms the top of the Inutile with 'the-exception of theportion which provides the iue cl.

As shown, the portion of the kiln between the furnacev c and theentrance, has the muiile walls so arranged as to set up varia tions inthe cross-sectional dimensions of the tunnel. For instance, in Fig. 2,it will be seen that the tunnel portion between the walls e aregradually spread apart so as to increase the dimensions between thewalls as a car advances from the entrance. At the entrance end, thedimensions are such as to practically receive the ladencar with butslight clearance, the spreadingof the walls in the horizontal plane,and, as shown in Fig. l, the increase in the height of the upper wall ofthe tunnel, increasing the clearance as the car advances, sufficientclearance being had to provide for the circulation action presentlyreferred to. This variation in the cross-sectional area of the tunnelmay be secured by forming a step or series of ste s, or by a gradualincrease or decrease, as y inclininfi or tapering the walls and crownlongitudinally of the tunnel.

The expanding tunnel-cross-section begins at the entrance end andcontinues until it approaches the neck c2 formed in the furnace where itis again contracted to provide for the small clearance conditions whichcon'- tinue to the exit end of the tunnel. The tunnel is therefore ofgreatest cross-sectional area where there is the greatest heat and isgradually contracted toward the cooler portions, so that in those partsof the tunnel where the greatest expansion of the air takes place, thetunnel will be correspondingly enlarged so thatthe internal air pressurein the tunnel will be substantially constant throughout the entirelength. Within this expanded portion of the tunnel we prefer to employbaffles f at the sides, these being preferably arranged staggered, andbaffles f at the top, as shown 1n Figs. 1 and 2, these balies being ofdimensions such as to permit of the free travel of the .cars through thetunnel, but which act to force the air to travel through the ware and tocirculatel as the cars advance in the heating portion of the tunnel.

As heretofore pointed out, the major feature of the present invention isthat which provides for thefiring of the ware in the presence ofcirculating air under pressure. This feature will bev more particularlyunderstood from the factthat at g is shown an air deliveringinstrumentality, such for instance, as a motor driven fan, the fan beingdesigned to deliver atmospheric air into the tunnel practically at itsexit end through a conduit g. At the entrance and the exit ends of thetunnel, and outside of lthe doors 10, we prefer to employ kvestibules h,it', these tending to close the entrance to and the exit from the tunnelwhen the doors 10 are open, the vestibule h receiving the car that is tobe added to the train and vestibule h receiving the car that is takenfrom the train when a car is added to the tra-in through the door fromvestibule h. This arrangement reduces theamount of air that might belost during the time when the train is being advanced by theintroduction of one car and the discharge of another car, the purposebeing to prevent, as far as possible, a loss of the air which is beingintroduced through channel g excepting such as is discharged through theopenings g2 located in the to walls of the mulle adjacent the entranceens of the kiln or which escapes around the entrance door, theseopenings g2 providing communication between the tunnel and flue al. Inthe installation heretofore referred to, the practice has been tointroduce the atmospheric air at the rate of approximately 500 cubicfeet perminute, which air is restricted in its escape at the entranceend of the tunnel, to provide internal air pressure in the tunnel.

As will be understood, the air introduced under the pressure of theinstrumentality g is being discharged mainly through the entrance oropposite end of the tunnel, and since the cooling -portion of thetunnel,

which extends from furnace c to the discharge end, is of a crosssectionaly dimension such as to provide for but a small clearance forthe car-the latter being more orA less skeleton in structure-the airis'forced to travel ractically through the car and thus bathe t e wareas well as the car structure with cool air as the air advances towardthe entrance end of the kiln and gradually becomes heated. It isobvious, of course, that since the ware is at its highest temperaturepracticallyat the point where the Vfurnace is located, the travel of theware in the direction opposite to that in which the air is being forced,sets up the conditions of theware, with result that the heat exchange isfrom the ware to the air. And, since this exchange is constantly actingto cool the ware, it will be readily understood that, at any crosssection of the tunnel within this cooling zone, the temperature of theair will be less than that of the ware, lalthough the difference intemperature between the ware and the air is comparatively small; forinstance, it may not exceed 100 F., and designedly does not exceed 200F.

The graph of the approximate conditions within this portion of the kilnis that shown at the right of Fig. 3, the temperature of the air beingindicated by the solid line m, the temperature of the ware beingrepresented by the dotted line n, which lines follow'each other veryclosely throughout the coolin or discharge end portion of the kiln, witht e temperature of air at a given distance from the end considerably.greater than that of the ware at the same distance throughout the mainportion of the entrance end and heating zone.

The variation limits as between the temperature of the air and that ofthe ware is maintained within the small compass shown by reason of theseconditions of heat exchange. The cold air, as it enters, bathes the wareon the car about to bedischarged, and as the temperature of the ware ishigher than that of the air-the latter being room temperature-the airabsorbs heat from the ware, and as the air advances to succeeding cars,its temperature has been increased by heat absorption but is stillcooler than the ware on these succeeding cars. This action-which lis theordinary heat exchange action-continues throughout the -length of thecooling zone, to more rapidly and uniformly reduce the temperature ofthe ware, and yet the action does not involve any sudden change in thetemperatures of either ware or air, the reduction of temperature of theware being more or less uniform, as is the increase of temperature ofthe air, the main factor in this being that the variations indifferences in temperatures between the ware and the air remainssubstantially uniform or is varied more or less regularly, so that thereis no sudden change in character of action produced upon the ware;

Upon reaching the furnace zone in its travel through the kiln, the airis at its highest temperature, since, at this portion the kilntemperature is greatest, as is also the temperature of the ware, and theair is caused by the battles to circulate in contact with the ware. Theware and the air thus reach approximately the same temperatures within tis zone, but as the air continues toward the entrance end of the kiln,its temperature'vhile being gradually lowered as it approaches the kiln,entrance, is still higher than that of the ware, the action thusproviding generally for the characteristic shown at the top of the graphof Fig. 3, in which there is a point where the two lines cross eachother, indicating that the temper'- ature of ware and air are equalapproximately atl this point.-

The advantage of this can be appreciated from the fact that althoughthis pointrep resents the point where the characteristics of thestresses change in the ware-the point where the action of expansion endsand that of contraction begins-the changes made under conditions wherethe air into /which the car is passing is almost of the same temperatureas that of the ware at the peak temperature of the latter, so that whilethere is the characteristics of a drop of temperature being forced,through the fact that the air, of less temperature than the ware, thedifference in temperature is so small as not to materially affect theconditions of stress and the change from stresses set up by expansion tothose set up by contraction are made so gradually and so naturally as topracticall eliminate all possibilities of shattering o the ware withinthis zone. As the car proceeds, the variation in temperature between thetwo may tend to increase, but the progression of increase in variationis so small as to be ineffective to set up changes which would affectthe contracting action in such manner as to set up shattering stresses.

It will be understood,of course, that the circulating action of the airwhich is caused by the baflies f and f also has the effect of bringingthe air into direct contact with the side walls e which tendto keep theair heated, without, however, materially affecting the radiating actionon the passing ware, set up by these walls. Hence this air which isgiving up its temperature through heat exchange action with theadvancing ware, is kept at a temperature above that of the ware throughthis heat exchange action with the walls e throughout the high heat zoneof the kiln. However, as the air becomes highly heated, it expands verymaterially, increasing its volume which would increase the internalpressure within the kiln and to prevent this increase of pressure at thehigh heat zone, this portion of the tunnel is increased incrosssectional area to correspond with the increase in expansion. Thisin" crease may be in 'one or more steps in the inner surface of 'thetunnel wall, as shown, or may be a gradual outward slanting of thewalls,and at those parts of the length of the tunnel where tlie airtemperature decreases and where there is therefore less expansionof theair, the tunnel walls arc drawnin any suitable construction to reducethe cross-sectional area and maintain uniform pressure.

Because of the air circulating feature within the zone to the left ofthe furnace in Flg. 2, it can be readily understood that there is moreor less similarity in variation between the temperature of the ware andthe air as is found on the cooling side. The variation may be of greaterextent, as indicated by the graph, but the limits of this variation arenot of such wide compass as would tendl to set up the conditions ofstress while the ware is within this critical or tender Zone. Like theconditions within the cooling zone, there may be differences `in theextent of the variation at spaced apart points` but these differencesare so small that they do not set up the conditions of sudden changesuch as are conducive to producing the shattering effect. The. airwithin this portion .of the tunnel is of a higher temperature than theware, and the two move in opposite directions through this portion ofthe kiln. Since the ware is of lower temperature than the air, the heatexchange is from the air to the ware, and this condition of heatexchange is present throughout the travel of this zone because of thefact that the air is maintained at this higher temperature because ofits own absorption of heat from the Walls e.

As shown at the left ofthe graph, there is a sudden drop in thetemperature of the air at the entrance end of the tunnel, this being dueto the fact that the openings g2 or leakage around the door permits ofthe escape of this air and the products of'combastion do-notreachte-this particular portion of the walls of the tunnel. This suddendrop, however, does not provide for any damaging action, for the reasonthat it takes place within the temperature range belowr that which endsthe critical temperature zone on a descending scale. While the Ware iswithin this critical zone, there is this approximate uniformity invariation such as to not produce any conditions of sudden change such aswould provide the stress action that produces shattering of the Ware.

There is present Within the tunnel, therefore` the presence of air withwhich the ware is being bathed, but this air is` a circulating air andnot a stagnant air. As will be understood, the introduction of 500 cubicfeet of air per minute into a tunnel kiln such as is presented herein,necessarily provides for an actual and positive movement of air in thedirection opposite that of the travel of the ware through the kiln. Notonly is there a constant pressure being exerted on the travellingr airthrough the increments of air being introduced, but the decreasingdimen-l sions of the tunnel at the entrance end of the latter` tend toset up conditions of resistance such as ,will place the entire body ofair within the ytunnel under a pressure condition. ln fact, the pressureof air Within the tunnel is designedly such as approach,

be equal to, or exceed the pressure within the product of combustionchannels, c', thus providing a resistance condition within the tunnelsuch as Will tend to prevent-leakage of products of combustion throughthe walls of the tunnel; and should such leakage take place despite thepresence 'of this counter pressure -or resistance, the small quantitiesthat might enter the tunnel through the walls of the tunnel would becomedissipated by reason of entering into a relatively large body of air,the latter travelling onward and not remaining at rest. V

The air pressure within the tunnel can be controlled by controllingleakagel around ings g2, by the use of the usual dampers that are wellknown in the art and which would overlie the openings. For instance, thenulnber of activeV openings might be reduced or might be increased byclosing` or opening some of these openings g2, this being a well knownexpedient in the art. It will be understood, of course, that the totalout-flow of air through these openings and through the door at theentrance end of the tunnel, approximates that which is being intro-lduced, While at the same time the conditions of pressure Within thetunnel will be presented. f-

' In practice, the volume of air being introduced remains substantiallyconstant if it is found that the temperature of the ware at the-peakzone is too high or too low, the remedy applied being that of changingthe heatudevelopment conditions within the furnace. If the temperatureof the ware be too,

high within the peak zone, the fuel consumption is cut down, While ifthe temperature of the`ware at the peak zone is below that which isdesired, the fuel supply is increased. This is referred to particularlyto point out the fact that the presence of the circulating air or theair which is being moved through the tunnel with its direction ofmovement opposite that of the ware is not accidental,

but is a real fundamental of the present invention. It will be noted, inthis connection, that when the 'installation that has heretofore beenreferred to is put into service'witnout the use of the flowing air, itis almost an invariable result that the Ware, or a major portion of it,will be shattered:l when, however, the kiln is operated with this airflow condition in which the air is constantly delivered., it constantlytravels and is under .a constant pressure within the kiln and in d1-rect contact with the ware, shattering of the ware is the exception andis exceedingly infrequent, a result that is directly contrary to thebeliefs and experiences of those who have been engaged in the art oftiring deco-r rated ceramic ware.

lt will be understood, of course, that the method provides for arelatively low cost .tunnel door or other escape such as the openofoperation with respect to fuel. This is the use of a fan therein, theair being circudue to the fact that the heated ware during lated wouldbe the same airtliis air "being the cooling operation isacting to heatup the equivalent of stagnant air, the movement the air to causetlielatter to itself act as a ofthe air within the kiln not changing thisheating agent for the Ware after the air has condition. passed the peakzone. Hence the amount Obviously, the labor required to operate of heatrequired to bring the ware to the a kiln under'tlie'inethod presentedherein desired temperatui'e is greatly decreased can be of the unskilledtype, whereas the through the fact that this heated air is actoperationwith the periodic type of inutile ing in conjunction With the radiatedheat kiln and .other types requires the use of from the walls of theinutile, the circulating skilled labor in order to prevent thecondiaction of the air not only serves to provide tions of shatteringtThis will be understood a more uniform heating of the ware, but from thefactthat the presence bf the air also tends to set up an action somewhatof travelling in the direction opposite the ware the t pe ofrecuperation of the heat of the and having a heat exchange actionbetween air through its being `brought into contact the two-travellingelements, tends to elimi- With the heated walls of the inutile. In othernate the conditions of sudden changes, suoli, words, the air, whenbrought tothe peak for instance, as might come from an untemperature asit passes through the cooling skilled workman attempting to replenishthe zone of the kiln, does-not lose its .ability to source of heatorcause the furnace to operheat With rapidity, due to the fact that itate at a higher heat. The heated air comis being constantly subjected tothe recupering from the cooling zone acts to 'tend to ating action setup by these walls as the air maintain the heat conditions, should thetravels onward, so that the loss of heating temperature of the productsof combustion abilityis a gradual one rather than sudden, seemingly bereduced, and the air will tend the air cooperating with the walls toproto take up excess heat from the Walls of vide a combined circulatingand radiating the mulle should the source of heat tend to activitywithiny the heating zone of the kiln. provide for overheating; the owingair As a result, the fuel requirements are greatly thus acts as 'anequalizer so as to maintain reduced and the cost of operation iscorregenerally a comparatively uniform action. spondingly reduced.Inpractice, pyrometer readii igs at different As compared with theactivities of the points on the-heating side adjacent the peak periodicmule kiln-the only one that has zone will give the operator a generalidea been employed under commercial production of the heat conditionswithin this particular conditions in connection with this third zone,and enable him to control the source stage of operation--the use of the`tunnel of heat, he knowing that with this control type of kiln underthe method of operation the action within the remainder of the kilnpresented herein provides another very will provide for the generaluniformity. material saving, not only as to time required As will beunderstood, ofcourse, the rate to complete the cycle, but also in thecharvof temperature reduction within the coolacter of the laboremployed. With the periing zone is somewhat higher than would odic type,it is necessary to introduce the be the rate if the air were notpresent. In ware into a cool kiln, then bring the latter other words,assuming a closed tunnel coolto its peak temperature, and then permitthe ing zone of the same length as is employed oven to cool, after whichthe ware is rein practicing the resent method, but in moved; theintroduction of air into an oven which the air supp y was omitted, andasof this type when used for this third stage suming the same ware, thesame peak temaction is practically barred in practice for peratureof'theware and the same rate of the reason that there is no way ofpreheatadvance of the ware, as, inthe practice of ing the air so as toavoid the wide variathe present invention, the ware would reach` tion intemperature between air and ware, the exit end of the kiln at a muchhigher it being readily understood that there is temperature than is thecase When practicing no circulation of fresh air possible within thismethod, the introduction of the air thus a kiln of this type so that atany particular increasing the rate of cooling above that of heatingtemperature, tlieair would vary the natural rate ofcooling such as wouldbe from that temperature by only 100 F. for provided in a tunnel withouttheI air supply; instance; any attempt to introduce air into to producea cooled ware of the l.same temthe kiln would be simply to increase theperature as is found at the exit end of the variation between air andware as the temtunnel in the present invention, would reperatui'e Withinthe kiln increased, and it quire an additional klength of the coolingwould not be possible to provide a' circulaso as t'o permit the ware toreach the desired tion within the kiln itself and have that temperature.-As a result,`the tunnel itself,

circulation provided with a constant supply in practicing the presentinventionhis of a of fresh air. Should an attempt be made materially'less length, thus requiring a to create a circulation within the kilnby shorter period for the completion of the cycle on any ware carried bya particular car so that there is an economy of time required tocomplete the cycle.

And this is true also in certain respects on the heating of the tunnel,it being possible to provide for an increasing rate of advance becauseof the fact that the variation between the air and theware is more orless uniform,`so that ware is not subjected to sudden changes. Inaddition, the fact that circulation is added to radiation as the heatingsource enables this result to be obtained for the reason that thecirculation brings the heated air to all parts of they car and itscontent, and hence does not produce a condition where the ware adjacentthe side walls of the mulhe become overheated while the ware in themiddle of the tunnel remains underheated, so that there is lessrequirement for a gradual rate of'advance, since'it is not necessary tomaintain the ware sufficiently long within a particular zone as to giveall vals parts of the car the opportunity of being brought practically4to the same heat.

It 1s to be understood, of course, that the underlying feature of thepresent invention is the discovery that ceramic ware can be brought toits peak zone for decorative firing while the ware is itself bathed withair circulated and under pressure. The entire development of thecommercial art has beeny contrary to this type of action. When theinstallation referred to was discussed with those experienced in thefiring of decorated ceramic ware, the opinion was universall expressedthat it was impossible to provi e commercial success. because the resultwould inevitably 4set up the most favorable conditions leading toshattering of the ware. Not only was this opinion refuted by the installation in action, but it has been found that the losses from spittingout, dunting over-firing or under-tiring, burned gold or gold improperlyset, as well as destruction or deterioration of gold or colors fromstagnant atmosphere, has been reduced to such` an extent. as to makethem relatively small, even as compared to the prior practice. Inaddition, the gold and the colors have la smoothness and bright, glossy`and lustrous appearance such as has heretofore been unobtainable in anyother type of mums kiln. As heretofore pointed out, the particularconstruction of the kiln shown in the drawings is illustrative only, thekiln shown bein simply representative of various types oy tunnel kilnsthat might be emfployed in practicing the method. A speci c constructionfor carrying out the method is shown the companion applicationheretofore identified. It will be understood, however, that while I wehave `herein shown and described one eneral way 1n which the presentlnventlon may be carried out, the invention is not debeing signed to belimited because of this particular disclosure, since the necessitv andexigen.- cies of a particular use or type of use may require variationsboth in structure and particular development of the method itself;because of these facts we desire-to be understood las reservingthe rightto make any and all such changes orl modiiications in the structure ormethod as may be found desirable or -essential in so far as the same mayfall within the spirit and scope of the in- -vention as expressed in theaccompanying claims when broadly construed.

What we claim is l. The art of firing articles in a kiln which consistsin subjecting the articles to a firing heat free from products ofcombustion and concurrently therewith to a flow of air in direct contactwith the articles or containers therefor.

2. The art of -iring articles in a kiln which consists in subjecting thearticles to a tiring heat free from products of combustion whilemaintaining a flow of air under pressure in direct contact with saidarticles,or containers therefor.

3. The art of firing articles in a kiln which consists in subjecting thearticles to a firing heat within the firing chamber of the kiln with theheat applied exteriorly of the ring chamber of the kiln and maintain thefiring chamber free from products of combustion,

and-concurrently therewith causing a flow of air fiin the tiring chamberpast the articles re I 4. The art of ring articles in a kiln whichconsists in subjecting the articles to a firing heat with the firingchamber of the kiln with the heat applied exteriorly of the firingchamber of the kiln and maintain the firing chamber substantially freeyfrom products of combustion and maintaining an air pressure `withinv'the firing chamber and a flow of air past the articles being Tired.

5. The art of tiring articles in a kiln which consists in subjecting thearticles to a firing heat withinthe firing chamber of the kiln with theheat ap lied exteriorly of the irmg chamber of the iln and maintain thefiring chamber substantially free from products of combustion, andmaintaining a circulation of air underpressure through the tiringchamber and around the articles being ire in the chamber, and in contactwith said articles.

6. The art of tiring articlesin a kiln which i ferent parts of thechamber, the articles bemg fired in said chamber in the presence of Saidair flow. 1

7. The art of firing articles in a kiln which consists in moving saidarticles in one direction throufgh the firing chamber of the kiln withthe ring chamber substantially free from products of combustion, and inproducing a flow of air through the chamber during the firing operationwith the direction of air flow opposite to the direction of movement'ofsaid articles.

8. The art of firing articles in a kiln having a comparatively long andnarrow firing chamber, and which consists in moving said articles in onedirection through the firing chamber, and producing a flow of airthrough the chamber in a direction opposite to that of the movement ofthe articles and under a substantially uniform pressure throughout thelength of tl e firing chamber, the air flow path being independent ofthe flow path of the products of combustion to prevent air contaminationby the products of combustion during passage of the air in contact withthe travelling articles.

9. The art of firing articles in a kiln which consists in passin thearticles through the firing chamber o the kiln in one direction andmaintaining a flow of air through the chamber during the firingoperation in a direction opposite to that of the movement of thearticles with the air flow maintained at a constant pressure throughoutlthe length of the chamber by compensating for the difference inexpansion of the traveling air at different parts of the chamber by avariation in the cross-sectional areal of the chamber.

10. In the art of decorating ceramic Ware, the method of rendering theapplied decorations permanent Which consists in subjecting the ware withthe applied decoration to a firing cycle while the ware is bathed withflowing air.

11. In the art of decorating ceramic ware, the method of rendering theapplied decorations permanent which consists in subjecting the ware withthe applied decoration to a firing cycle, and maintaining the warebathed with flowing air throughout the cycle.

12. In the art of decorating ceramic ware,

4the method of rendering the applied decorations permanent whichconsists in establishing a firing cycle travel path :for the ware, andmoving the ware with the applied decoration through such path inpresence of and in contact with flowing air.

13. In the art of decorating ceramic ware, the method of rendering theapplied decorations permanent which consists in establishing a firingcycle travel path-for the ware, and moving the ware with the applieddecoration through such path while maintaining the ware bathed withflowing air substantially throughout the period of traveLof the warethrough such path.

14. In the art of decorating ceramic Ware, the method of ,rendering theapplied "decorations permanent which consists in establishing a firingcycle travel path for the ware, moving the ware with the applieddecoration t rough such path, and flowing air through such pathconcurrently with the movement of the ware therethrough, the movement ofWare and air through the path being in opposite directions.

15. In the art of decorating ceramic ware, the method of rendering theapplied decorations permanent which consists in establishing a 'iringcycle travel path for the ware,

-rently with the movement of the ware therethrough.

16. In the art of decorating ceramic ware and wherein the ware with theapplied decoration is subjected to a firing cycle action in which thepeak temperature is found within the tender or critical temperature zoneof the ware, the method of rendering the applied decorations ermanentwhich consists in establishing a firing cycle travel path for 'the warein which the peak temperature zone is at an intermediate portion of thetravel path, moving the ware throughout the length of such travel path,and concurrently bathing the ware with flowing air.

v17. In the art of decorating ceramic ware and wherein the ware with theapplied decoration is subjected to a fir-mg c cle action in which thepeak temperature is ound within the tender or critical temperature zoneof the ware, the method 'of rendering the applied decorations permanentwhich' consists in establishing a firin cycle travel path for'the warein which t e peak temperature is at an intermediate portion of thetravel path, moving the ware throughout the length of such travel path,and concurrently establishing a flow of air in such path, with theflowing air bathing the ware.

18. In the art of decorating ceramic ware and wherein the ware with theapplied decoration is subjected to a firing cycle or criticaltemperature zone of the ware- Athe method of rendering the applieddecorations permanent whichconsists in establishing a ringv cycle travelpath for the ware in which the peak temperature zone is at anintermediate portion of the travel path, moving the ware throughout thelength of such travel path. and concurrently establishing a flow-of airin such path with the flowing air bathing the ware, the movement of theair `and ware through the path being in generally opposite directions.

19. In the art of decorating ceramic ware and wherein the ware with theapplied decoration is subjected to a firing cycle action in which thepeak temperature is found within the tender or critical temperature zoneof the warethe method of rendering the applied decorations permanentwhich consists in establishing a ring cycle travel path for the `Ware inwhich the peak temperature zone is at an'intermediate portion of thetravel path, moving the ware `throughout the length of such travel path,and concurrently establishing a flow of air in such path with theflowing air bathing the ware and with a portion of the air flow pathdeviating from a straight line path.

20. In the art of decorating ceramic ware and wherein the ware'with theapplied decoration is subjected to a firing cycle action in which thepeak temperature is found withinthe tender or critical temperature zoneof the ware, the method of rendering the applied decorations permanentwhich uconsists in establishing a firing cycle travel path for the warein which the peak temperature zone is-at an intermediate port-ion of thetravel path, moving the ware throughout the length of such travel pathand' concurrently establishing a flow of air in such path withAtheflovving air bathing the ware and with a portion of the air flowpath following a tortuous path through the ware.

21. In the artof decorating ceramic ware and wherein the Ware with theapplied decoration is subjected to a firing cycle action in which thepeak temperature is found withing the tender or critical temperatureZone of the ware, the method of rendering the applied decorationspermanent which consists in establishing a' firing cycle travel path'for the ware in 'which the peak temperature zone is at an intermediateportion of the travel path, moving the ware throughout the length ofsuch travel path, and concurrently establishing a flow of air in suchpath with, the flowing air bathing the ware and with the flow of airwithin that portion of the ware travel path which leads to the peaktemperature zone being generally in a tortuous path to r:ause airmovement angularly to the direction of travel of the ware.

22. Inthe art of decorating ceramic ware, wherein the ware with theapplied decoration is subjected .to a firing cycle in which the peaktemperature is found within the tender or critical temperature Zone ofthe ware, the method of rendering the applied decorations permanent bythe activities of the firing cycle which consists in subjecting the'ware' with the applied decoration to the complete firing cycle whilethe Ware is bathed with flowing air having its temperature varied ingeneral correspondence with the Itemperature variations of the ware.

23. In the art of decorating ceramic ware, wherein theware with theapplied decoration is subjected to a firing cycle in which the peaktemperature is found within the tender or critical temperature zone ofthe ware, the method of rendering the applied decorations permanent bythe activities of the firing cycle which consists in subjecting the warewith the applied decoration to the complete tiring cycle while the wareis bathed with flowing air having its temperature varied in generalcorrespondence with the temperature variations of the ware, and with thepeak temperatures of air and ware produced in approximate concurrence.

24. In the art of decorating ceramic ware, wherein the ware with theapplied decoration is subjected to a firing cycle in which the peaktemperature is found within the tender or critical temperature zone ofthe ware, .the method of rendering the applied decorations permanent bythe activities of ,the firing cycle which consists in subjecting theware with the applied decoration to' the complete fiiring cycle whilethe ware is bathed with flowing air having its temperature varied ingeneral correspondence with the temperature variations of the ware, andwith the temperatures of the air and ware of approximately equal valuewithin the peak temperature zone. f 25. In the art of decorating ceramicware, wherein the ware with the applied decoration is subjected to afiring cycle in which the peak temperature is found within the Aflowmovement vin a direction such as to p-roduce heat exchange activitybetween the air and ware at substantially all points of such vtravelpath beyond the peak temperature zone.

26. In the 'art of decorating ceramic ware, wherein the Ware with theapplied decoration is subjected to a firin j cycle in which the peaktemperature is .ound within tbe tender or critical temperature zone ofthe ware, themethod of rendering the applied decorations permanent bythe activities of the firing c cle which consists in establishing a irmgcycle travel path for the ware with the peak temperature zone of theware intermediate the ends of such travel'path, moving the ware throughsuch travel path, and concurrently establishing a ilowmove ment of airsubstantially through said travel path to bathe the ware, with the airflow movement in a general direction opposthe to ware in the directionof movement of the ware' to set 'up' the conditions of heat exchangebetween air and ware beyond the peak temperature zone, the differencesin temperature values of air and ware elective in producing the heatexchange activity being less than 200 F.

27. In the art of decorating ceramic ware, wherein the ware with theapplied decoration is subjected to a firing cycle in which the peaktemperature is found withinthe tender or critical temperature zone ofthe ware, the method of rendering the applied decorations permanent bythe-activities 4oi the liring cycle which consists in establishing aliring cycle travel path for the Ware with the peak temperature zone ofthe ware `intermediate the ends of such travel path, moving the warethrough such travel path, and concurrently estabhshing a flow movementof air substantially through said travel path to bathe the Ware, witht-he air flow movement in a general direction opposite the direction ofmovement of the ware to set up the conditions of heat exchange betweenair and ware beyond the peak temperature zone, with the heat exchangeactive from air portion lof the travel path in advance of the ak'zone,and from ware to air in the' portion of such path following the peakzone.

28, In the art of decorating ceramic ware, wherein the ware with the'applied decoration is subjected to a lirin cycle in which the peaktemperature is ound within the tender or critical temperature zone ofthe ware, the method of rendering the applied decorations permanent bythe activities of the firing cycle which consists in establishing afiring cycle travel path for the ware with the eak temperature zone ofthe ware intermediate the ends of such travel path, moving the warethrough such travel path, and concurrently establishing a flow movementof air substantially through said travel from air to ware in the portionof the travel path in advance of the peak zone, and from ware to air inthe portion of such path' following the peak zone, the dilerences intemperature values of air and ware effective in producing such heatexchange being less than 200 F. i

29. In the art of decorating ceramic ware wherein the ware with theapplied decoration is subjected to a firing cycle in which the peaktemperature is found within the tender or 'critical temperature zone ofthe ware, the method of rendering the applied decorations permanent bythe activities of thel liring7 cycle which consists in establishing atiring cycle travel path for the Ware with the peak temperature zone ofthe ware intermediate the ends of such travel path and with the travelpath of enlarged cross-sectional dimensions in advance of the peaktemperature zone, a zone leading from the entrance of decreasingcross-sectional dimensions, moving the ware throu h such travel path,and concurrently esta lishing an air flow path leading from adjacent thedischarge end of the ware travel path and traversing the ware travelpath into such entrance zone and discharged within such zone, the lairflow being such as to produce conditions of air pressure within suchflow path active'to counteract pressure of products of combustionapplied external of the travel path walls, the flowing air bathing the`ware travelling in the travel ath.

In testimony whereof we a x o'ur signatures.

CHARLES T. HOLCROFT. CHARLES E. DOLL.

